1. Field of the Invention
The invention relates to a novel locking mechanism for locking the piston rod, or shaft, of a fluid responsive actuator in at least one of two extreme axially spaced positions respectively corresponding to the open and closed positions of a valve member or the like manipulated by the actuator.
2. Description of the Prior Art
In oil fields, pipe lines, and refineries there has been a considerable need for a fluid actuated valve actuator which, in the event of an emergency causing the reduction or loss of control pressure supplied to the actuator, would cause the actuator to be mechanically locked in the position it held prior to the occurrence of the emergency. Depending upon the particular application, it may be desirable that the fluid operated valve actuator be mechanically locked in either its open or its closed position. More importantly, there are a significant number of valves requiring cycling operations between an open and closed position where it is desirable that such valves be locked in the same position as existed prior to the occurrence of the particular emergency. In a more common parlance, it is therefore desirable that a fluid responsive valve actuator be capable of being mechanically locked in a first position, a second position, or either a first or second position in the event of loss of, or reduction in, control pressure supplied to such valve actuator.
The invention constitutes an improvement in a locking mechanism for a fluid operated valve actuator described and claimed in U.S. patent application Ser. No. 944,050, filed Sept. 19, 1978, which is assigned to the assignee of this application.
In the aforementioned co-pending application Ser. No. 944,050, a separate fluid pressure responsive spring actuated locking mechanism independent of the main actuator piston is provided for securing the piston rod of a fluid operated actuator in either of the two extreme axially spaced positions. Such locking mechanism is a spring actuated secondary piston that locks the piston rod and main piston unit when low pressure exists, and is unlocked when higher pressure exists. Any pressure operated actuator is necessarily dependent upon the existence within the cylinder of a pressure differential across the actuator piston. It follows that a side of the piston which normally would be vented or connected to a lower pressure level could, because of the existence of unusual conditions, be exposed to a pressure sufficient to prevent fluid pressure responsive locking mechanism described in the aforesaid co-pending application Ser. No. 944,050 from locking the piston rod.
It has also been proposed in U.S. Pat. No. 2,221,121 to Wallace that a locking mechanism comprising a plurality of angular segments surrounding the piston rod be provided with camming surfaces which cooperate with similarly shaped surfaces on the piston rod, the camming surfaces being effective to move the locking segments radially outwardly on relative movement of the piston rod with respect thereto. To effect the locking of the piston rod, the annular locking segments are restrained against radially outward movement by an annular locking ring provided on a piston head which has a lost motion connection with the piston rod, and must be first moved to an unlocking position relative to the camming segments before the piston rod is enabled to cam the locking segments to their unlocked position. Both the piston head and piston rod must be moved before complete release of the locking segments is achieved, hence reacting a drag on the piston movement. In the locking sequence, the piston head and rod act in opposition on the locking segments, creating undesirable friction and the possibility that the locking segments may not lock.